You can remove them from any display groups they belong to. This will remove their name from the time clock worker list, but they'll still appear in all timecard report selection lists for admin users.
You can mark their user record as inactive or terminated. This will automatically remove them from any display groups they belong to, and remove them from all timecard report selection lists. The historical record of all hours and overtime worked will be maintained and can be easily accessed by simply making the user active again.
You can delete the user. This should be a last resort as all of the historical timecard entries, leave awards, and messages are completely and permanently removed. A hard copy of all hours for the worker should be printed before deletion. Deleting a user is irreversible, so use it with care.